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Hey Guys,
I just got though looking at my neighbor's 2007 F150 4.2L 2WD.
He had no brake lights.
First thing I checked were the fuses and looked to see if there were any relays (no relays).
There were 3 fuses: 1 on the main stop light circuit and 2 on the signal switch circuit.
Here's the reason I'm in here:
I want to know how to PROPERLY test FUSES.
Here's the back story:
Although all 3 fuses checked OK using an ohm reading, I still found that they were bad (at the end of the diagnosis).
The way I checked them was pulled each one from the socket and did an ohm reading on them. All zeroed out.
So I plugged them back in and then tested the brake pedal switch.
When that checked OK, I began to tear apart the dash to look for problems.
Besides the fact that there was a bunch of aftermarket garbage wired all up in everything (remote locks and electric windows), nothing else looked bad.
So I tore out all that garbage and then tested the multi-function switch (signal switch). which is part of the circuit.
The wire wasn't reading any supply voltage.
So then I went back to the fuse box with a DVM and tested each leg of the relevant fuses (probed the top metal tit).
One leg showed voltage, the other leg didn't.
So I got some old 10A fuses and replaced the 2 fuses related to the signal switch circuit.
What do you know? It worked!
So my question is about FUSES.
WHY did the fuses text OK with an ohm meter, but fail in the circuit?
What test can I use to FULLY TEST the fuses to make sure they're good?
I wasted 4 hours until I figured that out!
But at least I know most of the wiring and systems are good!
Thanx!
A plain voltage/voltage drop test will be your best friend.....remember to LOAD the circuit. The ohmmeter DOES NOT load the circuit and does not tell you anything about the current carrying capability of the circuit.....
Often times fuses are tested with an incandescent lest light connected to ground. For low amperage circuits this maybe enough, for high amperage circuits definitely it is not ...
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
Last edit: 2 years 10 months ago by juergen.scholl. Reason: I was in the spelling tunnel...
Thanx man. So basically I have to start at one end and work my way across the circuit. The problem is, I can't access any of the wires until I get to the fusee box.
This is just the beauty of the voltage and voltage drop tests, it exactly indicates where the problem is: the readings will be fine up to the point where the actual fault occurs.
Different strategies are possible, you may go to the half way point of the circuit and start your measurements there. That way you eliminate already 50% of everything in the circuit( if it`s a single fault 8-}) and go on from there the same way.... In other cases the accessibility may steer your testng approach. Many ways to skin a cat.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
